Wireless cellular communication networks are known. In these networks, the area covered by the network is divided into a number of cells. Each of the cells has associated with it a base transceiver station. The base transceiver stations are arranged to communicate with mobile devices located in the cells. The mobile devices can take any suitable form and are typically mobile telephones.
The need for efficient accurate positioning of the mobile telephones has increased. In the USA, network operators must be able to provide the location of mobile subscribers making emergency calls. Similar proposals are currently being considered in Europe. Additionally, commercial services such as tracking services (that is the emergency service mentioned above, people locating, valuable assets location etc), finding/guidance services (proximity services such as yellow pages, direction indicators, point of interest locators etc) and notification services (targeted advertisements, traffic alerts, weather alerts, bus/train location, guided tours etc) are currently being proposed.
In the GSM (global system for mobile communications) standard, four positioning methods have been included: cell identity and timing advance, time of arrival, enhanced observed time difference (E-OTD) and a method based on GPS (global positioning system technology either as standalone GPS or assisted GPS).
The time of arrival method is able to locate handsets with standard software but requires the installation of new network elements such as location measurement units at every base station site. The enhanced—observed time difference method requires the installation of location management units at every two to five base transceiver stations and a software modification in the handset. The assisted GPS method requires installation of a GPS receiver and possibly also location measurement units besides the integration of a GPS receiver into a handset. All of these methods require the introduction of a new network element or corresponding functionality responsible for location calculation called the serving mobile location centre SMLC.
The timely deployment of location services for all users including those having handsets which do not include the necessary software or hardware, requires that measurements already available in cellular networks should be used. These techniques are important as they allow operators and service providers to start offering location based services to all customers with minimal additional costs whilst waiting for more accurate and sophisticated location technologies to be available. From the technical point of view, even when technology such as enhanced-observed time difference and assisted GPS are fully available, network based software solutions will still be needed as backup methods when the new standardised solutions fail or when the requested accuracy can be met with such a method. Network based software techniques can also be used as an initial guess for the algorithms used to implement one of the standard solutions in order to improve the accuracy or speed of convergence of those algorithms.